1. Field of the Invention
The present invention relates to a wiring board having a via hole and a method of manufacturing the same.
2. Description of Related Art
As shown in FIG. 3, a wiring board 20 formed by electrically connecting a lower wiring conductor 11 and an upper wiring conductor 14 formed on an upper surface of an upper insulating layer 12 with a via conductor 13 that penetrates the upper insulating layer 12 is known (for example, see Japanese Unexamined Patent Application Publication No. 2002-252436). The via conductor 13 is formed by filling a via hole 15 formed in the upper insulating layer 12 with a plating conductor.
In recent years, in such a wiring board, a micro-wiring pattern is popularly used. For this reason, the via conductor 13 requires a diameter of 40 μm or less. However, when the via conductor 13 has a diameter of 40 μm or less, a contact area between the via conductor 13 and the lower wiring conductor 11 also decreases, and a contact strength therebetween is degraded. For this reason, for example, when heat generated in mounting parts on the wiring board 20 and heat generated in activation of the parts are repeatedly applied, thermal stress caused by a difference between the thermal expansion coefficients of the via conductor 13 and the lower wiring conductor 11 makes cracks between the via conductor 13 and the lower wiring conductor 11 to damage the electric connection between the via conductor 13 and the lower wiring conductor 11, then the wiring board 20 could not function as a wiring board.